Apparatus and methods for controlling led light flux

ABSTRACT

A rectangular pulse generator system is operatively configured to generate a generator output signal, the generator output signal formed as a base rectangular waveform gated by a modulating rectangular waveform, the base rectangular waveform having a first frequency and the modulating rectangular waveform having a second frequency less than the first frequency. A low-pass filter coupled to the rectangular pulse generator system is configured to receive a filter input signal representative of the generator output signal and to produce a filter output signal representative of the filter input signal. A voltage-controlled current source coupled to the low-pass filter generates a drive signal conducted by at least one LED producing a light flux determined by the current level of the LED drive signal. Methods are devised for calibration and for setting the average light flux level.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.62/402,514, filed Sep. 30, 2016, which application is incorporatedherein by reference in its entirety for all purposes.

BACKGROUND

Light flux refers to the total rate at which light is being emitted by alight source, and it may be expressed in terms such as radiant flux inunits of light energy per unit of time, photon or quantum flux in unitsof numbers of photons per unit of time, or luminous flux in units oflumens per unit of time.

In the art of lighting using LEDs (light-emitting diodes) as lightsources, various light flux setting systems exist, of which two basictypes may be described as follows. One type is the analog dimming type,in which a controlling electrical level, such as a voltage, is used toadjust the current that a driver circuit puts through one or more LEDs.At a particular light flux setting the amount of current through theLEDs may be more or less steady (DC) and approximately proportional tothe controlling electrical level. The light flux of the LEDs may beroughly proportional to the current through the LEDs and may thus bealso roughly proportional to the controlling electrical level.

An analog dimming type of light flux setting system may take advantageof the fact that, over a certain useful current range, LEDs generatelight more efficiently and last longer at lower currents than they do athigher currents. Systems that utilize highly efficient (˜85% or greater)switching converters to regulate the current through the LEDs mayoperate with high energy efficiency (radiant flux per electrical inputpower consumed) at a maximum light flux level and with even higherenergy efficiency at lower light flux levels down to, for example,twenty percent of the maximum light flux level. In addition, the LEDs insuch systems may, at lower light flux levels, maintain their performanceover operating periods many times longer than the lifetimes that theyexhibit when operating at maximum flux levels. Analog dimming may,therefore, produce energy-saving and lifetime-extending advantages inLED lighting systems operated at light flux levels substantially lowerthan the maximum light flux levels of which the systems are capable.Typically, a switching converter acting as an LED current driver underanalog control controls the current over a five-to-one or ten-to-onerange and turns the current off completely below the minimum of thatrange.

Another type of light flux setting system is a pulse-width-modulation(PWM) type, sometimes also referred to as a pulse-code modulation (PCM)type. This type of system sets an average light flux by allowing arectangular-waveform signal known as the PWM signal to turn the energysource on and off repeatedly at high speed with a duty cycle rangingbetween zero and one-hundred percent. With LEDs, the light emission maybe turned alternately fully on and fully off through modulation of thecurrent through the LEDs by the PWM signal.

As in analog dimming, a highly efficient switching converter may beutilized to regulate the current through the LEDs. Contrary to theanalog dimming approach, however, the PWM light flux setting systemoperates the LEDs at their maximum flux level during the part of thecycle in which the LEDs are fully on and is not designed to reduce thecurrent to non-zero levels below the current level required for themaximum flux level. As a result, a PWM light flux setting system in theexisting art generally does not take advantage of increased efficienciesthat can result from lower LED currents, and the perceived lifetimes ofthe LEDs are increased in inverse proportion to the duty cycle, but notas much as they would be if the light flux setting were accomplishedwith a reduction in current as in an analog dimming system. A PWM lightflux setting system may have advantages in terms of precise linearcontrol of the light flux, which light flux may be accuratelyproportional to the duty cycle of the PWM signal, and in terms ofstability of the wavelength spectrum of the LED, since this spectrum mayhave some dependence on the instantaneous current through the LED, whichcurrent is held constant during the maximum-current part of the PWMcycle. In addition, a PWM system typically can control average lightflux over a much wider range than can an analog dimming system. Thelight flux range is limited by the minimum pulse time over which maximumcurrent can be achieved in the driver and by the maximum period betweenpulses that can be allowed under flicker limitations.

SUMMARY

An apparatus and methods for controlling LED light flux are described.

In an example, an LED light flux setting system comprises a rectangularpulse generator system, a low-pass filter, a voltage-controlled currentsource, and at least one LED.

The rectangular pulse generator system is operatively configured togenerate a generator output signal, the generator output signal formedas a base rectangular waveform gated by a modulating rectangularwaveform, the base rectangular waveform having a first frequency and themodulating rectangular waveform having a second frequency less than thefirst frequency.

The low-pass filter has a cutoff frequency and is coupled to therectangular pulse generator system and configured to receive a filterinput signal representative of the generator output signal and toproduce a filter output signal representative of the filter input signalwith frequencies above the cut-off frequency being attenuated comparedto frequencies below the cutoff frequency.

The voltage-controlled current source is coupled to the low-pass filterand responsive to a control voltage signal representative of the filteroutput signal for generating an LED drive signal having a current levelrepresentative of a voltage level of the control voltage signal.

The at least one LED is configured to conduct the LED drive signal, theat least one LED producing a light flux determined by the current levelof the LED drive signal.

In another example, an LED light flux setting system comprises amicroprocessor, a low-pass filter, a voltage-controlled current source,and at least one LED.

The microprocessor is configured to generate a generator output signal,the generator output signal formed as a base rectangular waveform gatedby a modulating rectangular waveform, the base rectangular waveformhaving a first frequency more than 10 kHz and the modulating rectangularwaveform having a second frequency less than one-tenth of the firstfrequency, the microprocessor being controllable to vary a duty cycle ofthe base rectangular waveform and a frequency and duty cycle of themodulating rectangular waveform.

The low-pass filter has a cut-off frequency between the first frequencyand the second frequency and is coupled to the rectangular pulsegenerator system and configured to receive a filter input signalrepresentative of the generator output signal and produce a filteroutput signal representative of the filter input signal with frequenciesabove the cut-off frequency being attenuated compared to frequenciesbelow the cutoff frequency. The low-pass filter includes a capacitor anda resistive voltage divider, the resistive voltage divider applying aportion of a voltage of the filter input signal to the capacitor.

The voltage-controlled current source and at least one LED are similarto those of the first example.

In an example, an LED light flux setting method is devised comprisinggenerating, by a rectangular pulse generator system, a base rectangularwaveform having a first frequency and a first duty cycle; gating thebase rectangular waveform with a modulating rectangular waveform havinga second frequency less than the first frequency and a second dutycycle, the gated base rectangular waveform forming a generator outputsignal; filtering a filter input signal representative of the generatoroutput signal with a low-pass filter having a cutoff frequency toproduce a filter output signal representative of the filter input signalwith frequencies above the cut-off frequency being attenuated comparedto frequencies below the cutoff frequency; generating an LED drivesignal having a current level representative of a voltage level of acontrol voltage signal representative of the filter output signal; andproducing a light flux determined by the current level of the LED drivesignal by conducting the LED drive signal in at least one LED.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram of an example of avoltage-controlled current source supplying current to one or more LEDs.

FIG. 2 is a graph of an example of a current-versus-voltagecharacteristic of the voltage-controlled current source included in FIG.1.

FIG. 3 graphs light flux values at various currents for a typical LEDover its operating range and includes a quadratic curve fit to the datapoints.

FIG. 4 plots the approximate light-flux-versus-control-voltage responseof the circuit of FIG. 1 resulting from the typical characteristics inFIGS. 2 and 3.

FIG. 5 is a schematic block diagram of an example of a hybrid light fluxsetting system in which a rectangular pulse generator coupled with alow-pass filter is used to create a control voltage.

FIG. 6 shows the circuit diagram of an example of a simple R-C low-passfilter.

FIG. 7 shows the circuit diagram of an example of a two-stage R-Clow-pass filter.

FIG. 8A shows an example of a graph of simulation results demonstratingthe transformation of a PWM signal at the input of a low-pass filterinto an approximately DC voltage at the output of the filter when theduty cycle of the PWM signal is 90%.

FIG. 8B shows an example of a graph of simulation results demonstratingthe transformation of a PWM signal at the input of a low-pass filterinto an approximately DC voltage at the output of the filter when theduty cycle of the PWM signal is 20%.

FIG. 9 is a schematic block diagram of an example of a firstimplementation of a CPWM (compound pulse-width modulation) hybrid lightflux setting system.

FIG. 10 graphs example waveforms of the signals within the CPWMgenerator and at the output of the low-pass filter in the system of FIG.9.

FIG. 11A graphs examples of the modulating waveform and simulatedlow-pass filter output voltage in the system of FIG. 9 for operation ata modulation duty cycle of 90%.

FIG. 11B graphs examples of the modulating waveform and simulatedlow-pass filter output voltage in the system of FIG. 9 for operation ata modulation duty cycle of 6%.

FIG. 12 graphs examples of the modulating waveform and simulatedlow-pass filter output voltage in the system of FIG. 9 for operation ata modulating frequency half that of the modulating frequency used inFIG. 11B and with the modulation duty cycle equal to half that used inFIG. 11B.

FIG. 13 graphs examples of the same data as in FIG. 12, except with atwo-stage low-pass filter in place of the one-stage low-pass filter,resulting in a waveform at the filter output more accuratelyapproximating a rectangular waveform.

FIG. 14 is a schematic block diagram of an example of a secondimplementation of a CPWM hybrid light flux setting system featuring theuse of two rectangular waveform generators feeding an AND gate togenerate the CPWM signal.

FIG. 15 is a schematic block diagram of an example of a thirdimplementation of a CPWM hybrid light flux setting system featuring theuse of a microprocessor with a PWM output to generate the CPWM signal.

FIG. 16 is a schematic block diagram of an example of a fourthimplementation of a CPWM hybrid light flux setting system featuring theuse of a microprocessor with two PWM outputs to feed an AND gate andthereby generate the CPWM signal.

FIG. 17 is a schematic block diagram of an example of a preferredembodiment of a CPWM hybrid light flux setting system using amicroprocessor to generate the CPWM signal and including a voltagedivision capability in the low-pass filter.

FIG. 18 is a schematic block diagram of an example of a generalimplementation of a CPWM hybrid light flux setting system with theaddition of a user input device.

FIG. 19 is a flow chart of an example of a method for calibrating a CPWMhybrid light flux setting system.

FIG. 20 is a flow chart of an example of a method for setting variousaverage light flux outputs with a CPWM hybrid light flux setting system.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

The disclosed apparatus, architectures, algorithms, and methods for asystem controlling LED light flux will become better understood throughreview of the following detailed description in conjunction with thedrawings. The detailed description and drawings provide examples of thevarious embodiments described herein. Those skilled in the art willunderstand that the disclosed examples may be varied, modified, andaltered without departing from the scope of the disclosed structures.Many variations are contemplated for different applications and designconsiderations; however, for the sake of brevity, not every contemplatedvariation is individually described in the following detaileddescription.

In LED light sources there usually are limits to how low the operatingcurrent of the LEDs can be taken before the efficiency decreases or thelifetime of the LEDs decreases substantially or the light fluxes fromdifferent LEDs driven by the same current begin to vary unacceptablyfrom one LED to another.

Moreover, a switching converter may produce unacceptably inaccuratecurrent levels when operated at low current levels. Accurate sensing ofthe LED current in an electrically noisy switching environment requiresa current-sensing resistance high enough to drop a voltage well abovethe electrical noise level. Increasing the current-sensing resistance tomaintain sufficient voltage drop at low LED currents results inincreased power dissipation at higher LED currents. This higher powerdissipation causes a reduction in the efficiency of the switchingconverter. A tradeoff must be made between the current range and theefficiency.

Typically, a switching converter acting as an LED current driver underanalog control is limited to a current control range in the neighborhoodof five-to-one or ten-to-one.

An embodiment of a compound-PWM (CPWM) hybrid light flux setting systemis described in more detail with reference to FIGS. 1-20. In the variousfigures, like or similar features may have the same reference labels.Each figure may include one or more views of objects.

FIG. 1 shows a schematic block diagram of an example of ananalog-controlled light source 1. A voltage-controlled current source 2may supply an LED current I to an at least one LED 3. LED current I isdependent on a control voltage V present on an analog control input A ofvoltage-controlled current source 2.

The dependence of LED current I on control voltage V may be as shown bya current-versus-voltage graph 50 given by example in FIG. 2. At verylow control voltage V, LED current I may be essentially zero. As controlvoltage V increases and reaches a value V2 the LED current I may jump toa level proportional to value V2, where the proportionality constant maybe equal to the slope of a substantially linear portion 51 of therelationship between LED current I and control voltage V. At controlvoltage values between V2 and a saturation voltage V3 the LED current Imay remain proportional to control voltage V until control voltage Vreaches a saturation voltage V3 at and above which LED current I maybecome constant at a maximum current level IMAX. With descending controlvoltage V the LED current I may follow the same curve, except that thesubstantially linear portion 51 may continue for control voltage Vlevels between V2 and a level V1 at which LED current I may drop tosubstantially zero. The difference between V2 and V1, which is known inthe art as hysteresis, may be intentionally created in order to maintainstability in the presence of electrical noise when control voltage V isin the vicinity of levels V2 and V1.

A typical dependence of a light flux F emitted by the one or more LEDs 3on LED current I is plotted in a flux-versus-current graph 100 in FIG.3. Some markers 101 show light flux F values at various levels of LEDcurrent I as taken from a data sheet for a commercially available LED. Afitted flux-versus-current curve 102 graphs a relationship of the formF=A·I²+B·I in which constants A and B have been adjusted to reduce to asmall number the mean-square difference between the values given byflux-versus-current curve 102 and the values given by markers 101. Itcan be noted that flux-versus-current curve 102 may match markers 101with an accuracy typically better than a few percent.

Combining the LED current I dependence on control voltage V shown inFIG. 2 with the light flux F dependence on LED current I shown in FIG. 3results in a light flux F dependence on control voltage V shown by acontrol graph 150 in FIG. 4. Due to the near-linearity of thesubstantially linear portion 51 of the current-versus-voltage graph 50in FIG. 2, the nearly-quadratic relationship of light flux F versus LEDcurrent I shown in FIG. 3 is preserved as a quadratic portion 151 of thelight flux F as a function of control voltage V curve shown in FIG. 4.The relationship between light flux F and control voltage V over thequadratic portion 151 may be closely approximated, therefore, asF=C·V²+D·V, where F quantifies the light flux F, V quantifies thecontrol voltage V, and C and D are constants independent of V.

Useful values of the constants C and D may be determined frommeasurements of light flux F at two different control voltage points, avoltage V4 and a voltage V5, suitably chosen between voltages V1 and V3as shown in graph 150 of FIG. 4. Voltage V4 may be set close to voltageV2 to produce a light flux F4 moderately close to the minimumcontrollable level at control voltage level V1 but reliably achievable,and voltage V5 may be chosen to produce a light flux F5 moderately closeto but less than a maximum light flux level FMAX. The constants C and Dmay, for example, then be calculated uniquely as C=(F4/V4−F5/V5)/(V4−V5)and D=(F5−V4/V5−F4·V5/V4)/(V4−V5). With these values of C and D, then,the light flux F at any control voltage V between control voltage V2 andcontrol voltage V3 may be closely approximated by F=C·V²+D·V. Using theinverse of this relationship, the control voltage V required to achievea light flux F between a light flux level F2, associated with controlvoltage V2, and light flux level FMAX may be closely approximated byV=((1+4·C·F/D²)^(0.5)−1)·D/(2·C). Hence, the determination of thecontrol voltage settings V4 and V5 at two light flux levels F4 and F5respectively may result in a calibration from which the control voltageV required to produce approximately a desired light flux F within areachable range may be easily calculated.

The method just described for determining the control voltage V requiredto achieve a given reachable light flux F through the use of a quadraticcurve-fit approximation is simple and uses analytical solutions. It willbe clear to persons skilled in the art, however, that lower-order andhigher-order algebraic or polynomial curve fit equations may be usedinstead, or that transcendental equations, piecewise equations, or tablelook-ups may be used to approximate measured data taken at fewer or morepoints on the measured light-flux-versus-control-voltage curve. Also, itwill be clear that numerical, iterative, and/or table look-up methodsmay be used, where analytical solutions are unavailable or undesirable,to optimize the parameters for a curve fit and to find approximatevalues of control voltage V to achieve desired light flux values F. Inaddition, it will be clear that curve-fitting functions giving controlvoltage V in terms of light flux F may be used instead of functionsgiving light flux F in terms of control voltage V, thereby avoiding theneed to invert a function to determine a control voltage level V for adesired light flux value F.

FIG. 5 shows a block diagram of an example of a hybrid light fluxsetting system 200 in which a rectangular pulse generator 201 cascadedwith a low-pass filter 202 is used to create the control voltage V atthe analog control input A of voltage-controlled current source 2 inanalog-controlled light source 1.

Rectangular pulse generator 201 may be a PWM generator capable ofproducing a signal with a desired frequency and a variable duty cycle.

Low-pass filter 202 may be a simple R-C (resistor-capacitor) filter suchas a simple R-C filter 250 shown in FIG. 6 having a series resistor 251,a parallel capacitor 252, a filter input node 253, a filter output node254, and an electrical ground node 255. Series resistor 251 may beelectrically connected at one of its two ends to filter input node 253and at its other end to filter output node 254, and parallel capacitor252 may be electrically connected at one of its two ends to filteroutput node 254 and at its other end to electrical ground node 255.

Alternatively, low-pass filter 202 may be an L-C (inductor-capacitor)filter (not shown), a multi-stage R-C filter such as a two-stage R-Cfilter 300 as shown in FIG. 7, or an active or passive filter of a lessor more complex type as is well known in the art.

Two-stage R-C filter 300 may include a first resistor 301, a firstcapacitor 302, a second resistor 303, and a second capacitor 304. Firstresistor 301 may be electrically connected at one of its two ends tofilter input node 253 and at its other end to an intermediate node 305,and second resistor 303 may be electrically connected at one of its twoends to intermediate node 305 and at its other end to filter output node254. First capacitor 302 may be electrically connected at one of its twoends to intermediate node 305 and at its other end to electrical groundnode 255, and second capacitor 304 may be electrically connected at oneof its two ends to filter output node 254 and at its other end toelectrical ground node 255.

The term “node” used in previous paragraphs and in the remainder of thisdescription may be defined as a point in a circuit, to which point oneor more terminals of circuit elements may be electrically connected andhave substantially identical electrical potential or voltage.

Shown in FIGS. 8A and 8B are a first graph 350 and a second graph 351 ofa general voltage VG versus time T. In first graph 350 a first locus 352plots a rectangular waveform with a duty cycle of 90% generated byrectangular pulse generator 201, and a second locus 353 plots an exampleof the resulting steady-state control voltage V at the analog controlinput A of voltage-controlled current source 2 in the hybrid light fluxsetting system 200 of FIG. 5. In this example, low-pass filter 202 maybe the simple R-C filter 250, as shown in FIG. 6, with a particular R·Ctime constant equal to 7.48 times the period of the rectangular waveformwith first locus 352. In this example it may be assumed that the outputimpedance of rectangular pulse generator 201 is negligibly small andthat the input impedance at analog control input A is high enough topresent a negligible load to filter output node 203 of low-pass filter202. As depicted in FIG. 8A by second locus 353, the steady-statecontrol voltage V resulting from the 90% duty cycle may be anapproximately DC (direct current) voltage equal to approximately 90% ofa peak voltage VPEAK of the rectangular waveform plotted by first locus352.

In second graph 351 in FIG. 8B a third locus 354 plots a rectangularwaveform with a duty cycle of 20% generated by rectangular pulsegenerator 201, and a fourth locus 355 plots the resulting steady-statecontrol voltage V at the analog control input A of voltage-controlledcurrent source 2. In this example, all conditions other than the dutycycle may be assumed to be unchanged from the conditions associated withfirst graph 350 in FIG. 8A. Fourth locus 355 demonstrates that thesteady-state control voltage V resulting from the 20% duty cycle may bean approximately DC voltage equal to approximately 20% of the peakvoltage VPEAK of the rectangular waveform plotted by third locus 354.

In general, for any duty cycle ranging from 0% to 100%, the averagevoltage of the approximately DC control voltage V in the hybrid lightflux setting system 200 of FIG. 5 under the conditions described abovemay be substantially equal to the duty cycle times the peak voltageVPEAK of the rectangular waveform and may therefore be a somewhatpredictable and approximately linear function of duty cycle. With ahybrid light flux setting system 200 as diagrammed in FIG. 5 thetechniques described previously for calibration and for setting controlvoltage V to achieve a desired light flux may be applied equally wellwhen PWM duty cycle is used in place of control voltage V as thecontrolling variable.

FIG. 9 shows, as an example, a first implementation 400 of a CPWM hybridlight flux setting system, in which a second rectangular pulse generator401 has been added to hybrid light flux setting system 200 of FIG. 5. Asecond output 402 of second rectangular pulse generator 401 is connectedto a modulation input M on rectangular pulse generator 201. Themodulation activated by modulation input M may be such that whenever thesignal at second output 402 of second rectangular pulse generator 401 issubstantially at its peak, the signal at a first output 403 ofrectangular pulse generator 201 is substantially the same as wasdescribed previously with reference to FIGS. 5 and 8, and whenever thesignal at second output 402 of second rectangular pulse generator 401 issubstantially at its minimum, the voltage at the first output 403 ofrectangular pulse generator 201 is substantially zero.

A modulation result graph 450 in FIG. 10 plots three voltages over timein a particular case to demonstrate an example of the operation of firstimplementation 400. A modulation locus 451 plots the voltage versus timeat second output 402 of second rectangular pulse generator 401. In theparticular case shown, modulation locus 451 has a PWM duty cycle of 50%.A modulated locus 452 plots the voltage versus time at first output 403of rectangular pulse generator 201. In the particular example shown,rectangular pulse generator 201 operates at a frequency equal to twentytimes the frequency of modulation locus 451 and has a PWM duty cycle of20%. A filtered result locus 453 plots the voltage versus time at filteroutput node 203 of low-pass filter 202. In the particular case shown asan example, low-pass filter 202 is assumed to be a simple R-C filter 250as shown in FIG. 6 in which the R·C time constant is 7.48 times theperiod of rectangular pulse generator 201, which period is defined asthe reciprocal of the frequency at which rectangular pulse generator 201is operates.

It will be observed that, during times T when modulation locus 451 is atpeak voltage VPEAK, filtered result locus 453 rises toward the steadystate shown by fourth locus 355 in FIG. 8 and that, during the timeperiods when the voltage shown by modulation locus 451 is at zero,filtered result locus 453 falls toward zero.

FIGS. 11A and 11B show, in a 90%-modulation-duty-cycle-graph 500 and a6%-modulation-duty-cycle-graph 501 respectively, examples of the resultsthat may be achieved when the frequency of second rectangular pulsegenerator 401 is set to one two-thousandth of the frequency setting ofrectangular pulse generator 201 with no other changes, other thanchanges in duty cycle, relative to the situation graphed in FIG. 10.

In the 90%-modulation-duty-cycle-graph 500 in FIG. 11A, a 90% modulationlocus 502 plots the voltage at second output 402 of second rectangularpulse generator 401 when the duty cycle of second rectangular pulsegenerator 401 is set to 90%. The resulting waveform at filter outputnode 203 is shown by a 90% result locus 503. The 90% result locus 503represents approximately a rectangular waveform with a peak amplitudeVCTL equal to 20% of peak voltage VPEAK and with a duty cycle of 90%.When presented to analog control input A of voltage-controlled currentsource 2 as shown in FIG. 9, this rectangular waveform may act topulse-width modulate the 20%-of-maximum LED current I thatvoltage-controlled current source 2 drives through the one or more LEDs3 when a steady voltage equal to peak amplitude VCTL is presented toanalog control input A. The average light flux from the LEDs will thusbe approximately 90% of the light flux emitted at a steady LED current Iof 20% of the maximum current IMAX (see FIG. 2).

If the duty cycle of second rectangular pulse generator 401 is droppedto lower values, the average light flux from the LEDs will dropaccordingly. In 6%-modulation-duty-cycle-graph 501 in FIG. 11B, a 6%modulation locus 505 plots the voltage at second output 402 of secondrectangular pulse generator 401 when the duty cycle of secondrectangular pulse generator 401 is 6%. The resulting waveform at filteroutput node 203 is shown by a 6% result locus 506. The 6% result locus506 represents approximately a rectangular waveform with peak amplitudeVCTL equal to 20% of peak voltage VPEAK and with a duty cycle of 6%.When presented to analog control input A of voltage-controlled currentsource 2 as shown in FIG. 9, this rectangular waveform may act topulse-width modulate the 20%-of-maximum LED current I thatvoltage-controlled current source 2 drives through the one or more LEDs3 when a steady voltage equal to peak amplitude VCTL is presented toanalog control input A. The average light flux from the LEDs will thusbe approximately 6% of the light flux emitted at a steady LED current Iof 20% of the maximum current IMAX (see FIG. 2).

It will be clear to persons skilled in the art that the average lightflux from the LEDs will be substantially proportional to the duty cycleof second rectangular pulse generator 401 so long as the resultingwaveform at filter output node 203 closely approximates a rectangularwaveform.

It will also be clear that the approximation to a rectangular waveformbecomes poor when the width of the pulses at the second output 402 ofsecond rectangular pulse generator 401 becomes too small. Deviationsfrom rectangularity are starting to become significant in the 6% resultlocus 506 shown in 6%-modulation-duty-cycle-graph 501. Further reductionof the duty cycle, and hence the pulse width, of second rectangularpulse generator 401 may, in fact, result in pulses at filter output node203 that fall significantly short of peak amplitude VCTL. To preventthis deviation from linearity, the narrowing of the pulse width ofsecond rectangular pulse generator 401 as duty cycle is decreased shouldstop at a point short of the point at which unacceptable deviations fromrectangularity in the waveform at filter output node 203 may occur.Further reductions in the duty cycle of second rectangular pulsegenerator 401 may then be achieved through reduction of the frequency ofthe pulses from second rectangular pulse generator 401.

A graph 550 in FIG. 12 shows as an example a result that may occur whenthe frequency of second rectangular pulse generator 401 is reduced tohalf of the frequency featured in FIG. 11B. A 3% modulation locus 551plots the voltage at the output of second rectangular pulse generator401, which now has a duty cycle of 3%. A 3% result locus 552 shows anexample of a resulting waveform at filter output node 203 that, to thesame degree as the waveform of FIG. 11B, approximates a rectangularwaveform, but now with a 3% duty cycle. So long as the pulse width fromsecond rectangular pulse generator 401 remains constant, the duty cyclecan be set arbitrarily low through reduction of the frequency of secondrectangular pulse generator 401. As will be clear to persons skilled inthe art, the duty cycle will be an accurate linear function of thereciprocal of this frequency.

The results demonstrated in FIGS. 11B and 12 may be improved through theuse of filters of higher order than that of simple R-C filter 250 (FIG.6). In FIG. 13 an improved result graph 600 shows an example of resultsfrom first implementation 400 (FIG. 9) with all parameters unchangedfrom those chosen for FIG. 12, except for the replacement of simple R-Cfilter 250 with two-stage R-C filter 300 (FIG. 7) to act as low-passfilter 202. The values of the components within two-stage R-C filter 300in the example of improved result graph 600 are 5,500 ohms for firstseries resistor 301, 1275 pF for first shunt capacitor 302, 16,500 ohmsfor second series resistor 303, and 425 pF for second shunt capacitor304. Comparing an improved 3% result locus 601 in FIG. 13 to the 3%result locus 552 in FIG. 12 may demonstrate how two-stage R-C filter 300with the given component values may yield a 3% result locus that moreclosely approximates a rectangular waveform with 3% duty cycle. At somecost in complexity, therefore, the linearity of LED current as afunction of the duty cycle of second rectangular pulse generator 401 maybe made more accurate, or an existing degree of linearity may bepreserved down to lower duty cycle limits.

It will be clear to persons skilled in the art that similar improvementsmay also be achieved with simple R-C filter 250 acting as low-passfilter 202 if, for example, the frequency setting of rectangular pulsegenerator 201 is increased and the R·C time constant of simple R-Cfilter 250 is decreased in proportion to the square root of the periodof rectangular pulse generator 201. Practical limitations, however,including limitations on the speed and accuracy of rectangular pulsegenerator 201 and problems created by parasitic reactances in thecircuitry, may limit the maximum frequency to which rectangular pulsegenerator 201 can be set without impairment of performance results.

The technique of modulating a PWM generator with another PWM generatorto produce waveforms of the type exemplified by modulated locus 452 inFIG. 10, and the types underlying FIGS. 11A through 13, may be termedcompound pulse-width modulation (CPWM). The combination of secondrectangular pulse generator 401 and rectangular pulse generator 201connected to each other as shown in FIG. 9, may be considered to be arectangular pulse generator system capable of generating a CPWM signalat first output 403.

There are many ways in which a CPWM generator capable of controlling ahybrid light flux setting system 200 (FIG. 5) may be architected. FIG.14 shows a block diagram of a second implementation 650 of a CPWM hybridlight flux setting system. The CPWM generator in this implementationcomprises a high-frequency PWM generator 651 and a low-frequency PWMgenerator 652 each connected to a separate input of an AND gate 653. Aswill be clear to persons skilled in the art, AND gate 653 as connectedin second implementation 650 may act as a 100% amplitude modulator, andthe waveform at AND gate output 654 may be of the type exemplified bymodulated locus 452 in FIG. 10.

The combination of high-frequency PWM generator 651, low-frequency PWMgenerator 652, and AND gate 653 all connected to each other as shown inFIG. 14, may be considered to be a rectangular pulse generator systemcapable of generating a CPWM signal at its output 654.

FIG. 15 shows a block diagram of a third implementation 700 of a CPWMhybrid light flux setting system. A microprocessor 701 with a PWM output702 may be programmed with internal timers to turn the signal at PWMoutput 702 on and off at substantially arbitrary times, therebysubjecting PWM output 702 to 100% amplitude modulation. Manycommercially available microprocessors have a built-in capability forgenerating PWM signals without the use of CPU (central processing unit)resources. Such a microprocessor may be set to output a PWM signal ofsubstantially arbitrary, within wide limitations, frequency and dutycycle at an output terminal such as PWM output 702. Such amicroprocessor may also contain timers that may be programmed to turnthe PWM output on and off at substantially arbitrary times under CPUcontrol and thereby generate a CPWM signal. In some cases, amicroprocessor may have the capability to generate two PWM signals andto have one of these PWM signals turn on and off the output of theother, thereby modulating it. Such an arrangement may require little orno CPU involvement. At the other extreme a microprocessor without aninternal PWM generator but with a digital output and a timing capabilitymay be programmed to output a CPWM signal by way of suitably timedcommands from the CPU to transition the output between ones and zeros.

Microprocessor 701 configured and programmed as described above withreference to FIG. 15 may be considered to be a rectangular pulsegenerator system capable of generating a CPWM signal at its output 702.

FIG. 16 shows a block diagram of a fourth implementation 750 of a CPWMhybrid light flux setting system. A microprocessor with dual PWM outputs751, including a first PWM output 752 and a second PWM output 753 mayhave each of these outputs connected to one of the inputs of AND gate653. The result at AND gate output 654 may be the same as in secondimplementation 650 in FIG. 14. Fourth implementation 750 has theadvantage that it may be applied to generate CPWM signals with no CPUinvolvement through use of a microprocessor that can automatically(without CPU involvement) generate two PWM outputs though it cannotprovide internally for automatic modulation of one of those outputs byanother.

The combination of the microprocessor with dual PWM outputs 751 and ANDgate 653 connected to each other as shown in FIG. 14, may be consideredto be a rectangular pulse generator system capable of generating a CPWMsignal at its output 654.

FIGS. 9, 14, 15, and 16 show examples of CPWM generators usable forcontrolling a hybrid light flux setting system 200, but it will be clearto persons skilled in the art that there also exist other types ofelectronic circuitry and waveform generators not shown that are capableof generating the described CPWM signals.

A preferred embodiment of a CPWM hybrid light flux setting system may bedescribed as follows. With reference to FIG. 17, a preferred embodiment800 may include a voltage-controlled current source 2 having a currentoutput I linearly controllable with a control voltage V ranging from of0.2 to 1.5 volts at analog control input A, which driver may beconnected to drive the at least one LED 3. Analog control input A mayhave an input impedance exceeding 1 megohm. Voltage-controlled currentsource 2 may have a response time, defined as the time required for LEDcurrent I to settle to within one percent of a new current outputsetting in response to a change in control voltage V, of less than 100microseconds.

Preferred embodiment 800 may also include a low-pass filter 202comprising an input resistor 801 with resistance 11,000 ohms, a dividerresistor 802 with resistance 11,000 ohms, and an output shunt capacitor803 with capacitance 6800 pF. Input resistor 801 may be electricallyconnected at one of its two ends to filter input node 253 and at itsother end to filter output node 203. Divider resistor 802 may beelectrically connected at one of its two ends to filter output node 203and at its other end to electrical ground node 255. Output shuntcapacitor 803 may be electrically connected at one of its two ends tofilter output node 203 and at its other end to electrical ground node255. Filter output node 203 may be connected to analog control input A.

Further included in preferred embodiment 800 may be microprocessor 701operating at a clock speed of, for example, 16 MHz and having anautomatic PWM generator outputting a PWM waveform at PWM output 702 witha frequency fbase equal to 200 kHz and an arbitrary duty cycle Dbase.PWM output 702 may be connected to filter input node 253. Microprocessor701 may be powered by a power supply (not shown) regulated at 3.3 volts.Microprocessor 701 may have a CMOS (complementarymetal-oxide-semiconductor) output stage at PWM output 702 with outputresistance less than 100 ohms both for sourcing of current and forsinking of current. The peak voltage of the signal at PWM output 702 maybe substantially equal to 3.3 volts, and the minimum voltage of thesignal at PWM output 702 may be substantially equal to 0.0 volts.

Microprocessor 701 may be programmed to modulate PWM output 702 byturning the PWM signal on and off at an arbitrary modulation frequencyfmod and an arbitrary duty cycle Dmod. When the PWM signal is off, PWMoutput 702 may be at zero volts. The resultant signal at PWM output 702may thus be a CPWM signal with peak amplitude 3.3 volts.

Low-pass filter 202 may act both as a two-to-one voltage divider and asan R-C filter with an R·C time constant of 37.4 microseconds. Thevoltage at filter output node 203 may range from zero volts to 1.65volts, depending on the duty cycle Dbase of the automatic PWM generatorthe modulated signal from which is presented at PWM output 702.

A more general implementation 850 of a CPWM hybrid light flux settingsystem is shown as an example in FIG. 18. It may include a rectangularpulse generator system 851 operatively configured to generate a CPWMoutput signal, a low-pass filter 202 coupled to the rectangular pulsegenerator system 851 and configured to receive a filter input signalrepresentative of the generator output signal, and an analog-controlledlight source 1. Analog-controlled light source 1 may comprise avoltage-controlled current source 2, having an analog control input A,and one or more LEDs 3 the LED current I through which is provided as adrive signal by the voltage-controlled current source 2. Thevoltage-controlled current source 2 may be coupled through its analogcontrol input A to the filter output node 203 of low-pass filter 202.

A user input device 852 may be coupled to rectangular pulse generatorsystem 851 to allow user or sensor input to select values of controlvariables that may include modulation frequency fmod, modulation dutycycle Dmod, and the duty cycle Dbase and frequency fbase of the PWMsignal being modulated. The user input device 852 may be a computer, asmartphone, a terminal, or any other type of device capable ofresponding to stimuli—such as user inputs, sensor signals, or automatedcommands—and controlling rectangular pulse generator system 851. Thecoupling between the user input device 852 and the rectangular pulsegenerator system 851 may be wireless or hard-wired.

The LED light flux characteristics of a CPWM hybrid light flux settingsystem may be calibrated, providing a light sensor is available that hasa known response to the LED light flux. A flow chart for an example of acalibration procedure is shown in FIG. 19.

For example, the LED light flux characteristics of preferred embodiment800 may be calibrated as follows. The frequency fbase of the PWM signalbeing modulated may be set to 200 kHz, and modulation frequency fmod maybe set to 200 hertz. Modulation duty cycle Dmod may be set to 100%. Dutycycle Dbase may then be adjusted to achieve an LED light flux F,measured by the light sensor, equal to a maximum guaranteed LED lightflux of value F1 for the system. The value of duty cycle Dbase resultingfrom this adjustment may be recorded as D1. Duty cycle Dbase may then beset to a value of D2=20%, and the consequent LED light flux value F2,measured by the light sensor, may be recorded. Two constants G and H maythen be calculated as G=(F1/D1−F2/D2)/(D1−D2) andH=(F2·D1/D2·F1·D2/D1)/(D1−D2). The values of two constants J=H/(2·G) andK=G/H² may then be calculated and stored along with LED light flux valueF2 in microprocessor 701's nonvolatile memory. These stored values ofconstants J and K and LED light flux F2 may constitute the calibrationconstants of the system.

In operation, various LED light flux settings may be achieved asdetailed, for example, in the following paragraphs. FIG. 20 shows a flowchart applicable to this example.

For any LED light flux value F greater than F2, the modulation dutycycle Dmod may be set to 100%, and the duty cycle Dbase may be set tothe lesser of 1 or Dset1=J·((1+4·K·F.)^(0.5)−1). This case may be termedcontrol mode 1.

For any LED light flux value F ranging from LED light flux F2 down toLED light flux X·F2, where in this example X=0.9, the duty cycle Dbasemay be frozen at D2=20%, the modulation duty cycle Dmod may be set tothe value Dset2=F/F2, and the modulation frequency fmod may be set tofset2=(1−Dset2)/T, where T in this example is 500 microseconds. Thiscase may be termed control mode 2.

For any LED light flux value F ranging from light flux X·F2 down tolight flux Y·F2, where in this example Y=0.1, the duty cycle Dbase mayremain frozen at D2=20%, the modulation frequency fmod may be set to avalue fset3, which in this example equals 200 Hz, and the modulationduty cycle Dmod may be set to the value Dset3=F/F2. This case may betermed control mode 3.

For any LED light flux value F ranging from light flux Y·F2 down toarbitrarily low average light flux values, duty cycle Dbase may remainat D2=20%, the modulation duty cycle Dmod may be set to the valueDset4=F/F2, and the modulation frequency fmod may be set tofset4=Dset4/T, where T in this example is 500 microseconds. This casemay be termed control mode 4.

Finally, for an LED light flux value F that is not greater than zero, itis sufficient to either set the modulation duty cycle Dmod to zeroand/or to set the duty cycle Dbase of the automatic PWM generator tozero. This case may be termed control mode 5.

Altogether, in this scheme there are five control modes. The rationalebehind this five-mode approach is as follows.

Control mode 1 uses an analog control method to dim the LEDs. Advantageis taken of the fact that the efficiency, defined as the light flux perunit of electrical power consumed, of the at least one LED 3 and thevoltage-controlled current source 2 taken together rises as the LEDcurrent I through the at least one LED 3 drops from its highest leveldown to about 20% of the highest level. In this first control mode thecontrol variable is the duty cycle Dbase of the PWM generator inmicroprocessor 701, and the light flux as a function of this controlvariable fits substantially accurately a quadratic relationship that canbe inverted to calculate the control variable value required to producea desired light flux. The other four control modes keep the operatingcurrent of the LEDs at the high-efficiency 20% level. The 20% level maybe sufficiently above the low end of the range of operating currentsprescribed by the LED manufacturer for reliable and consistent operationof the LEDs.

At the maximum guaranteed LED light flux of value F1 the calibratedvalue of Dset1 may be less than 100%, since the at least one LED 3 maybe more efficient in some instances than in others. The method ofcontrol mode 1 may accommodate settings of LED light flux F greater thanF1 producing accurate responses so long as the calculated value of Dset1remains no higher than 100%. If the user-derived setting of LED lightflux F is so high that the calculated value of Dset1 would exceed 100%,Dset1 is limited to exactly 100%, which may produce the maximum LEDlight flux F of which the system is capable.

Control mode 2 pulse-code modulates the 20%-of-maximum current,periodically turning it off for a time period T=500 microseconds. Thisoff-time period is long enough to allow both the driver and the low-passfilter output to settle sufficiently to prevent significantresponse-time-related errors in the average light flux. The modulationfrequency fmod in this control mode varies from arbitrarily low valuesup to a maximum of 200 hertz. Flicker, which can be annoying to humans,begins to become discernible when light flux is turned on and off at amodulation frequency fmod below 200 hertz. However, when the off time isonly 500 microseconds and the average dimming caused by the modulationis no more than 10%, the flicker may be imperceptible. In the example ofthe preferred embodiment, the average dimming at a modulation frequencyfmod of 100 hertz will be 5% and at 50 hertz will be only 2.5%. Flickershould be insignificant.

In control mode 3 the modulation frequency fmod remains constant at 200hertz while the modulation duty cycle changes. Flicker is avoided byvirtue of the 200 hertz modulation frequency. The low end of the averagelight flux range in this control mode occurs when the modulation pulsewidth falls to 500 microseconds, below which response times might affectthe accuracy of the average light flux settings.

In control mode 4 the modulation duty cycle depends on modulationfrequency fmod, which drops below 200 hertz to continue the reduction inaverage light flux while maintaining the modulation pulse width at 500microseconds. A shortcoming of this control mode is that flicker becomespronounced as the light flux is further reduced. In some applications,however, such as the provision of light for photosynthesis of plants,the flicker may be inconsequential.

In control mode 5 the intention is to turn the at least one LED 3 offcompletely so that the LED light flux is zero. The intention isaccomplished if the duty cycle of either the base PWM generator or themodulator is set to zero so that no pulses are generated.

Overall, the five-mode hybrid analog/PWM LED light flux setting schemewith the settings and component values described offers accurate averagelight flux settings over a 50-to-1 flicker-free dimming range and over asubstantially infinite dimming range when perceptible flicker isallowed. The code for calculating and setting the modulation frequencyfmod, the modulation duty cycle Dmod, and the microprocessor's automaticPWM duty cycle Dbase to achieve a user-specified light flux F may beprogrammed into the microprocessor 701, rendering the manipulationsinvisible to the user and seemingly instantaneous.

The LED light flux setting system described takes advantage of theimproved efficiency that analog control may provide at moderate dimminglevels and also retains the advantages of high linearity and extendeddimming range that PWM may provide. It provides for calibration of theLED light flux so that the flux at any dimming level may be constantfrom one light source to another despite unit-to-unit variations inlight source performance. It also allows the user to set the LED lightflux to values above the maximum guaranteed LED light flux value F1 toachieve LED light fluxes up to the maximum level of which the system iscapable. Additionally, the LED light flux setting system describedminimizes flicker in the light source, so that flicker is negligibleover a wider range of average LED light flux values than can be coveredwith pure PWM control.

It will be understood by persons skilled in the art that many variationsin the operational aspects of this LED light flux setting scheme may becontemplated. The cross-over points between control phases may bechanged, maximum frequencies and response time allowances may change,the low-pass filter design and order may be changed, the criteria to bemet for flicker-free lighting may be changed, the CPWM generation schememay be changed, the calibration or curve-fitting method may be altered,and/or there may be other changes not mentioned. Depending on accuracy,flicker, and dimming range requirements or latitudes, one or more of thecontrol phases may be eliminated completely or additional control phasesmay be added.

CPWM hybrid light flux setting systems described herein may be appliednot just to LED lighting control, but also, with modifications, to motorcontrol, power control, or the control of other items. CPWM hybrid lightflux setting systems may be particularly advantageous in applications inwhich the item being controlled operates more efficiently at low analogcontrol levels than at high control levels.

Adjustments may be added to the LED light flux setting system tocompensate for variables such as operating temperature and age. Forinstance, a microprocessor that generates and/or controls the CPWMsignal for setting the LED light flux may include a temperature sensor,and the microprocessor may make use of the measured temperature and theflux-versus-temperature characteristics of the LEDs to appropriatelyadjust the target light flux level F to be achieved by the LED lightflux setting system and to thereby correct for temperature variations.

Accordingly, while embodiments have been particularly shown anddescribed, many variations may be made therein. Other combinations offeatures, functions, elements, and/or properties may be used. Suchvariations, whether they are directed to different combinations ordirected to the same combinations, whether different, broader, narrower,or equal in scope, are also included.

The remainder of this section describes additional aspects and featuresof a compound-PWM hybrid LED light flux setting system presented withoutlimitation as a series of paragraphs, some or all of which may bealphanumerically designated for clarity and efficiency. Each of theseparagraphs can be combined with one or more other paragraphs, and/orwith disclosure from elsewhere in this application, including thematerials incorporated by reference, in any suitable manner. Some of theparagraphs below expressly refer to and further limit other paragraphs,providing without limitation examples of some of the suitablecombinations.

A1. An LED light flux setting system comprising:

a rectangular pulse generator system operatively configured to generatea generator output signal, the generator output signal formed as a baserectangular waveform gated by a modulating rectangular waveform, thebase rectangular waveform having a first frequency and the modulatingrectangular waveform having a second frequency less than the firstfrequency;

a low-pass filter having a cutoff frequency, the low-pass filter coupledto the rectangular pulse generator system and configured to receive afilter input signal representative of the generator output signal andbeing configured to produce a filter output signal representative of thefilter input signal with frequencies above the cut-off frequency beingattenuated compared to frequencies below the cutoff frequency;

a voltage-controlled current source coupled to the low-pass filter andresponsive to a control voltage signal representative of the filteroutput signal for generating an LED drive signal having a current levelrepresentative of a voltage level of the control voltage signal; and

at least one LED configured to conduct the LED drive signal, the atleast one LED producing a light flux determined by the current level ofthe LED drive signal.

A2. The LED light flux setting system of paragraph A1, wherein therectangular pulse generator system is controllable to vary the secondfrequency of the modulating rectangular waveform.

A3. The LED light flux setting system of paragraph A1, wherein themodulating rectangular waveform has pulses with a second duty cycle, andthe rectangular pulse generator system is controllable to vary thesecond duty cycle.

A4. The LED light flux setting system of paragraph A1, wherein therectangular pulse generator system is controllable to vary the firstfrequency of the base rectangular waveform.

A5. The LED light flux setting system of paragraph A1, wherein the baserectangular waveform has pulses with a first duty cycle, and therectangular pulse generator system is controllable to vary the firstduty cycle.

A6. The LED light flux setting system of paragraph A1, wherein thelow-pass filter has a cut-off frequency below the first frequency.

A7. The LED light flux setting system of paragraph A1, wherein thelow-pass filter has a cut-off frequency above the second frequency.

A8. The LED light flux setting system of paragraph A6, wherein therectangular pulse generator system includes a base rectangular pulsegenerator for generating the base rectangular waveform, the baserectangular pulse generator being responsive to the modulatingrectangular waveform for gating the base rectangular waveform.

A9. The LED light flux setting system of paragraph A8, wherein therectangular pulse generator system further includes a modulatingrectangular pulse generator for generating the modulating rectangularwaveform.

A10. The LED light flux setting system of paragraph A1, wherein therectangular pulse generator system includes an AND gate, a baserectangular pulse generator coupled to a first input of the AND gate,and a modulating rectangular pulse generator coupled to a second inputof the AND gate, the base rectangular pulse generator is configured togenerate the base rectangular waveform, the modulating rectangular pulsegenerator is configured to generate the modulating rectangular waveform,and the AND gate is responsive to the base rectangular waveform and themodulating rectangular waveform for producing the generator outputsignal.

A11. The LED light flux setting system of paragraph A1, wherein therectangular pulse generator system includes a microprocessor configuredto generate the generator output signal.

A12. The LED light flux setting system of paragraph A1, wherein therectangular pulse generator system includes a microprocessor configuredto generate at least one of the base rectangular waveform and themodulating rectangular waveform.

A13. The LED light flux setting system of paragraph A12, wherein themicroprocessor is configured to generate both the base rectangularwaveform and the modulating rectangular waveform, and the rectangularpulse generator system further includes an AND gate responsive to thebase rectangular waveform and the modulating rectangular waveform forproducing the generator output signal.

A14. An LED light flux setting system comprising:

a microprocessor configured to generate a generator output signal, thegenerator output signal formed as a base rectangular waveform gated by amodulating rectangular waveform, the base rectangular waveform having afirst frequency more than 10 kHz and the modulating rectangular waveformhaving a second frequency less than one-tenth of the first frequency,the microprocessor being controllable to vary a duty cycle of the baserectangular waveform and a frequency and duty cycle of the modulatingrectangular waveform;

a low-pass filter having a cut-off frequency between the first frequencyand the second frequency, the low-pass filter coupled to the rectangularpulse generator system and configured to receive a filter input signalrepresentative of the generator output signal and produce a filteroutput signal representative of the filter input signal with frequenciesabove the cut-off frequency being attenuated compared to frequenciesbelow the cutoff frequency, the low-pass filter including a capacitorand a resistive voltage divider, the resistive voltage divider applyinga portion of a voltage of the filter input signal to the capacitor;

a voltage-controlled current source coupled to the low-pass filter andresponsive to a control voltage signal representative of the filteroutput signal for generating an LED drive signal having a current levelrepresentative of a voltage level of the control voltage signal; and

at least one LED configured to conduct the LED drive signal, the atleast one LED producing a light flux determined by the current level ofthe LED drive signal.

A15. The LED light flux setting system of paragraph A14, wherein themicroprocessor is configured to operate in a first mode in which theduty cycle of the base rectangular waveform is controllable and the dutycycle and frequency of the modulating rectangular waveform are constant,and at least a second mode in which the duty cycle of the baserectangular waveform and frequency of the modulating rectangularwaveform are held constant and the duty cycle of the modulatingrectangular waveform is controllable.

A16. The LED light flux setting system of paragraph A15, wherein the atleast a second mode includes a third mode, and the frequency of themodulating rectangular waveform is different in the second mode and thethird mode.

B1. An LED light flux setting method comprising:

generating, by a rectangular pulse generator system, a base rectangularwaveform having a first frequency and a first duty cycle;

gating the base rectangular waveform with a modulating rectangularwaveform having a second frequency less than the first frequency and asecond duty cycle, the gated base rectangular waveform forming agenerator output signal;

filtering a filter input signal representative of the generator outputsignal with a low-pass filter having a cutoff frequency to produce afilter output signal representative of the filter input signal withfrequencies above the cut-off frequency being attenuated compared tofrequencies below the cutoff frequency;

generating an LED drive signal having a current level representative ofa voltage level of a control voltage signal representative of the filteroutput signal; and

producing a light flux determined by the current level of the LED drivesignal by conducting the LED drive signal in at least one LED.

B2. The LED light flux setting method of paragraph B1, furthercomprising:

receiving by the rectangular pulse generator one or more inputsrepresentative of intended values of the first duty cycle, the secondduty cycle, and the second frequency; and

setting the values of the first duty cycle, the second duty cycle, andthe second frequency in response to the received one or more inputs.

B3. The LED light flux setting method of paragraph B2, furthercomprising:

provision by a processor to the rectangular pulse generator of an inputrepresentative of an intended second-duty-cycle value of 100%;

operation by the processor to find and store in memory, for each of oneor more predetermined time-averaged-light-flux-calibration values, avalue of the first duty cycle that, when set, causes the time-averagedlight flux measure provided by a sensor to have approximately thetime-averaged-light-flux-calibration value;

operation by the processor to, for each of one or more predeterminedfirst-duty-cycle-calibration values, provide an input to the rectangularpulse generator to cause the value of the first duty cycle to be set tothe first-duty-cycle-calibration value and to, once the first duty cycleis set, store the resulting time-averaged light flux measure provided bythe sensor; and

operation by the processor to calculate and store in memory, using theone or more predetermined time-averaged-light-flux-calibration values,the one or more stored values of the first duty cycle, the one or morepredetermined first-duty-cycle-calibration values, and the one or morestored time-averaged light flux measures, one or more fitting constantsthat the processor can subsequently use, possibly along with one or morepredetermined constants, to determine an approximate setting of thefirst duty cycle that will result in a prescribed obtainable numericalmeasure from the sensor of the time-averaged light flux produced by theat least one LED.

B4. The LED light flux setting method of paragraph B3, wherein thenumber of values of fitting constants stored by the processor is two andwherein the approximate setting of the first duty cycle is determinedfrom the inverse of a quadratic relationship, which quadraticrelationship relates the numerical measure provided by the sensor to thevalue of the first duty cycle and gives a numerical measure of zero whenthe first duty cycle is zero.

B5. The LED light flux setting method of paragraph B2, furthercomprising:

receiving by a processor an input representative of an intended value oftime-averaged light flux;

calculation by the processor, using stored values of fitting constants,of a calculated first-duty-cycle value that, when set as the value ofthe first duty cycle while the second duty cycle is 100%, should resultin production of a time-averaged light flux by the at least one LEDapproximately equal to the intended value of time-averaged light flux;

calculation by the processor of a limited first-duty-cycle value equalto 100% if the calculated first-duty-cycle value is greater than 100%,equal to a predetermined minimum value less than 100% if the calculatedfirst-duty-cycle value is less than the predetermined minimum value, orequal to the calculated first-duty-cycle value if the calculatedfirst-duty-cycle value not greater than 100% and not less than thepredetermined minimum value;

provision by the processor to the rectangular pulse generator of aninput representative of an intended first-duty-cycle value the same asthe limited first-duty-cycle value; and,

if the calculated first-duty-cycle value is not less than the prescribedminimum value, provision by the processor to the rectangular pulsegenerator of an input representative of an intended second-duty-cyclevalue of 100%.

B6. The LED light flux setting method of paragraph B5, furthercomprising:

calculation by the processor, either from one or more stored values oftime-averaged light flux measure or using the stored values of thefitting constants, the time-averaged light flux value F2 expected whenthe first duty cycle is set to the predetermined minimum value and thesecond duty cycle is set to 100%;

determination by the processor of a Boolean result, the Boolean resultbeing true if the intended value of time-averaged light flux is lessthan time-averaged light flux value F2 and no less than a predeterminedfraction X of time-averaged light flux value F2, and the Boolean resultbeing false otherwise;

performance of the following operations if, and only if, the Booleanresult is true;

calculation by the processor of a calculated second-duty-cycle valueequal to the intended value of time-averaged light flux divided bytime-averaged light flux value F2;

calculation by the processor of a calculated second-frequency valueobtained by dividing a predetermined minimum time-period value into thedifference between 100% and the calculated second-duty-cycle value; and

provision by the processor to the rectangular pulse generator of aninput representative of an intended second-duty-cycle value the same asthe calculated second-duty-cycle value and an input representative of anintended second-frequency value the same as the calculatedsecond-frequency value.

B7. The LED light flux setting method of paragraph B5, furthercomprising:

calculation by the processor, either from one or more stored values oftime-averaged light flux measure or using the stored values of thefitting constants, the time-averaged light flux value F2 expected whenthe first duty cycle is set to the predetermined minimum value and thesecond duty cycle is set to 100%;

determination by the processor of a Boolean result, the Boolean resultbeing true if the intended value of time-averaged light flux is lessthan a predetermined fraction X of time-averaged light flux value F2 andno less than a predetermined fraction Y of time-averaged light fluxvalue F2, and the Boolean result being false otherwise;

performance of the following operations if, and only if, the Booleanresult is true;

calculation by the processor of a calculated second-duty-cycle valueequal to the intended value of time-averaged light flux divided bytime-averaged light flux value F2; and

provision by the processor to the rectangular pulse generator of aninput representative of an intended second-duty-cycle value the same asthe calculated second-duty-cycle value and an input representative of anintended second-frequency value the same as a predetermined referencesecond-frequency value.

B8. The LED light flux setting method of paragraph B5, furthercomprising:

calculation by the processor, either from one or more stored values oftime-averaged light flux measure or using the stored values of thefitting constants, the time-averaged light flux value F2 expected whenthe first duty cycle is set to the predetermined minimum value and thesecond duty cycle is set to 100%;

determination by the processor of a Boolean result, the Boolean resultbeing true if the intended value of time-averaged light flux is greaterthan zero and less than a predetermined fraction Y of time-averagedlight flux value F2, and the Boolean result being false otherwise;

performance of the following operations if, and only if, the Booleanresult is true;

calculation by the processor of a calculated second-duty-cycle valueequal to the intended value of time-averaged light flux divided bytime-averaged light flux value F2;

calculation by the processor of a calculated second-frequency valueequal to the calculated second-duty-cycle value divided by apredetermined minimum time-period value;

provision by the processor to the rectangular pulse generator of aninput representative of an intended second-duty-cycle value the same asthe calculated second-duty-cycle value and an input representative of anintended second-frequency value the same as the calculatedsecond-frequency value.

B9. The LED light flux setting method of paragraph B5, furthercomprising:

determination by the processor of a Boolean result, the Boolean resultbeing true if the intended value of time-averaged light flux is lessthan or equal to zero, and the Boolean result being false otherwise;

performance of the following operation if, and only if, the Booleanresult is true;

provision by the processor to the rectangular pulse generator of aninput representative of an intended first-duty-cycle value of zero or aninput representative of an intended second-duty-cycle value of zero.

INDUSTRIAL APPLICABILITY

The methods and apparatus described in the present disclosure areapplicable to the general lighting industry, the decorative lightingindustry, the specialty lighting industry, the agricultural lightingindustry, the horticultural lighting industry, the research lightingindustry, the military lighting industry, and all other industries inwhich LEDs or other electrically-powered sources are employed to producelight. They are also applicable to other industries in which items areto be controlled electrically and can benefit from an accurateimplementation of analog control combined with pulse-code modulationcontrol.

What is claimed is:
 1. An LED light flux setting system comprising: arectangular pulse generator system operatively configured to generate agenerator output signal, the generator output signal formed as a baserectangular waveform gated by a modulating rectangular waveform, thebase rectangular waveform having a first frequency and the modulatingrectangular waveform having a second frequency less than the firstfrequency; a low-pass filter having a cutoff frequency, the low-passfilter coupled to the rectangular pulse generator system and configuredto receive a filter input signal representative of the generator outputsignal and being configured to produce a filter output signalrepresentative of the filter input signal with frequencies above thecut-off frequency being attenuated compared to frequencies below thecutoff frequency; a voltage-controlled current source coupled to thelow-pass filter and responsive to a control voltage signalrepresentative of the filter output signal for generating an LED drivesignal having a current level representative of a voltage level of thecontrol voltage signal; and at least one LED configured to conduct theLED drive signal, the at least one LED producing a light flux determinedby the current level of the LED drive signal.
 2. The LED light fluxsetting system of claim 1, wherein the rectangular pulse generatorsystem is controllable to vary the second frequency of the modulatingrectangular waveform.
 3. The LED light flux setting system of claim 1,wherein the modulating rectangular waveform has pulses with a secondduty cycle, and the rectangular pulse generator system is controllableto vary the second duty cycle.
 4. The LED light flux setting system ofclaim 1, wherein the rectangular pulse generator system is controllableto vary the first frequency of the base rectangular waveform.
 5. The LEDlight flux setting system of claim 1, wherein the base rectangularwaveform has pulses with a first duty cycle, and the rectangular pulsegenerator system is controllable to vary the first duty cycle.
 6. TheLED light flux setting system of claim 1, wherein the low-pass filterhas a cut-off frequency below the first frequency.
 7. The LED light fluxsetting system of claim 1, wherein the low-pass filter has a cut-offfrequency above the second frequency.
 8. The LED light flux settingsystem of claim 6, wherein the rectangular pulse generator systemincludes a base rectangular pulse generator for generating the baserectangular waveform, the base rectangular pulse generator beingresponsive to the modulating rectangular waveform for gating the baserectangular waveform.
 9. The LED light flux setting system of claim 8,wherein the rectangular pulse generator system further includes amodulating rectangular pulse generator for generating the modulatingrectangular waveform.
 10. The LED light flux setting system of claim 1,wherein the rectangular pulse generator system includes an AND gate, abase rectangular pulse generator coupled to a first input of the ANDgate, and a modulating rectangular pulse generator coupled to a secondinput of the AND gate, the base rectangular pulse generator isconfigured to generate the base rectangular waveform, the modulatingrectangular pulse generator is configured to generate the modulatingrectangular waveform, and the AND gate is responsive to the baserectangular waveform and the modulating rectangular waveform forproducing the generator output signal.
 11. The LED light flux settingsystem of claim 1, wherein the rectangular pulse generator systemincludes a microprocessor configured to generate the generator outputsignal.
 12. The LED light flux setting system of claim 1, wherein therectangular pulse generator system includes a microprocessor configuredto generate at least one of the base rectangular waveform and themodulating rectangular waveform.
 13. The LED light flux setting systemof claim 12, wherein the microprocessor is configured to generate boththe base rectangular waveform and the modulating rectangular waveform,and the rectangular pulse generator system further includes an AND gateresponsive to the base rectangular waveform and the modulatingrectangular waveform for producing the generator output signal.
 14. AnLED light flux setting system comprising: a microprocessor configured togenerate a generator output signal, the generator output signal formedas a base rectangular waveform gated by a modulating rectangularwaveform, the base rectangular waveform having a first frequency morethan 10 kHz and the modulating rectangular waveform having a 70 secondfrequency less than one-tenth of the first frequency, the microprocessorbeing controllable to vary a duty cycle of the base rectangular waveformand a frequency and duty cycle of the modulating rectangular waveform; alow-pass filter having a cut-off frequency between the first frequencyand the second frequency, the low-pass filter coupled to the rectangularpulse generator system and configured to receive a filter input signalrepresentative of the generator output signal and produce a filteroutput signal representative of the filter input signal with frequenciesabove the cut-off frequency being attenuated compared to frequenciesbelow the cutoff frequency, the low-pass filter including a capacitorand a resistive voltage divider, the resistive voltage divider applyinga portion of a voltage of the filter input signal to the capacitor; avoltage-controlled current source coupled to the low-pass filter andresponsive to a control voltage signal representative of the filteroutput signal for generating an LED drive signal having a current levelrepresentative of a voltage level of the control voltage signal; and atleast one LED configured to conduct the LED drive signal, the at leastone LED producing a light flux determined by the current level of theLED drive signal.
 15. The LED light flux setting system of claim 14,wherein the microprocessor is configured to operate in a first mode inwhich the duty cycle of the base rectangular waveform is controllableand the duty cycle and frequency of the modulating rectangular waveformare constant, and at least a second mode in which the duty cycle of thebase rectangular waveform and frequency of the modulating rectangularwaveform are held constant and the duty cycle of the modulatingrectangular waveform is controllable.
 16. The LED light flux settingsystem of claim 15, wherein the at least a second mode includes a thirdmode, and the frequency of the modulating rectangular waveform isdifferent in the second mode and the third mode.
 17. An LED light fluxsetting method comprising: generating, by a rectangular pulse generatorsystem, a base rectangular waveform having a first frequency and a firstduty cycle; gating the base rectangular waveform with a modulatingrectangular waveform having a second frequency less than the firstfrequency and a second duty cycle, the gated base rectangular waveformforming a generator output signal; filtering a filter input signalrepresentative of the generator output signal with a low-pass filterhaving a cutoff frequency to produce a filter output signalrepresentative of the filter input signal with frequencies above thecut-off frequency being attenuated compared to frequencies below thecutoff frequency; generating an LED drive signal having a current levelrepresentative of a voltage level of a control voltage signalrepresentative of the filter output signal; and producing a light fluxdetermined by the current level of the LED drive signal by conductingthe LED drive signal in at least one LED.
 18. The LED light flux settingmethod of claim 17, further comprising: receiving by the rectangularpulse generator one or more inputs representative of intended values ofthe first duty cycle, the second duty cycle, and the second frequency;and setting the values of the first duty cycle, the second duty cycle,and the second frequency in response to the received one or more inputs.19. The LED light flux setting method of claim 18, further comprising:provision by a processor to the rectangular pulse generator of an inputrepresentative of an intended second-duty-cycle value of 100%; operationby the processor to find and store in memory, for each of one or morepredetermined time-averaged-light-flux-calibration values, a value ofthe first duty cycle that, when set, causes the time-averaged light fluxmeasure provided by a sensor to have approximately thetime-averaged-light-flux-calibration value; operation by the processorto, for each of one or more predetermined first-duty-cycle-calibrationvalues, provide an input to the rectangular pulse generator to cause thevalue of the first duty cycle to be set to thefirst-duty-cycle-calibration value and to, once the first duty cycle isset, store the resulting time-averaged light flux measure provided bythe sensor; and operation by the processor to calculate and store inmemory, using the one or more predeterminedtime-averaged-light-flux-calibration values, the one or more storedvalues of the first duty cycle, the one or more predeterminedfirst-duty-cycle-calibration values, and the one or more storedtime-averaged light flux measures, one or more fitting constants thatthe processor can subsequently use, possibly along with one or morepredetermined constants, to determine an approximate setting of thefirst duty cycle that will result in a prescribed obtainable numericalmeasure from the sensor of the time-averaged light flux produced by theat least one LED.
 20. The LED light flux setting method of claim 19,wherein the number of values of fitting constants stored by theprocessor is two and wherein the approximate setting of the first dutycycle is determined from the inverse of a quadratic relationship, whichquadratic relationship relates the numerical measure provided by thesensor to the value of the first duty cycle and gives a numericalmeasure of zero when the first duty cycle is zero.
 21. The LED lightflux setting method of claim 18, further comprising: receiving by aprocessor an input representative of an intended value of time-averagedlight flux; calculation by the processor, using stored values of fittingconstants, of a calculated first-duty-cycle value that, when set as thevalue of the first duty cycle while the second duty cycle is 100%,should result in production of a time-averaged light flux by the atleast one LED approximately equal to the intended value of time-averagedlight flux; calculation by the processor of a limited first-duty-cyclevalue equal to 100% if the calculated first-duty-cycle value is greaterthan 100%, equal to a predetermined minimum value less than 100% if thecalculated first-duty-cycle value is less than the predetermined minimumvalue, or equal to the calculated first-duty-cycle value if thecalculated first-duty-cycle value is not greater than 100% and not lessthan the predetermined minimum value; provision by the processor to therectangular pulse generator of an input representative of an intendedfirst-duty-cycle value the same as the limited first-duty-cycle value;and, if the calculated first-duty-cycle value is not less than theprescribed minimum value, provision by the processor to the rectangularpulse generator of an input representative of an intendedsecond-duty-cycle value of 100%.
 22. The LED light flux setting methodof claim 21, further comprising: calculation by the processor, eitherfrom one or more stored values of time-averaged light flux measure orusing the stored values of the fitting constants, the time-averagedlight flux value F2 expected when the first duty cycle is set to thepredetermined minimum value and the second duty cycle is set to 100%.determination by the processor of a Boolean result, the Boolean resultbeing true if the intended value of time-averaged light flux is lessthan time-averaged light flux value F2 and no less than a predeterminedfraction X of time-averaged light flux value F2, and the Boolean resultbeing false otherwise; performance of the following operations if, andonly if, the Boolean result is true; calculation by the processor of acalculated second-duty-cycle value equal to the intended value oftime-averaged light flux divided by time-averaged light flux value F2;calculation by the processor of a calculated second-frequency valueobtained by dividing a predetermined minimum time-period value into thedifference between 100% and the calculated second-duty-cycle value; andprovision by the processor to the rectangular pulse generator of aninput representative of an intended second-duty-cycle value the same asthe calculated second-duty-cycle value and an input representative of anintended second-frequency value the same as the calculatedsecond-frequency value.
 23. The LED light flux setting method of claim21, further comprising: calculation by the processor, either from one ormore stored values of time-averaged light flux measure or using thestored values of the fitting constants, the time-averaged light fluxvalue F2 expected when the first duty cycle is set to the predeterminedminimum value and the second duty cycle is set to 100%; determination bythe processor of a Boolean result, the Boolean result being true if theintended value of time-averaged light flux is less than a predeterminedfraction X of time-averaged light flux value F2 and no less than apredetermined fraction Y of time-averaged light flux value F2, and theBoolean result being false otherwise; performance of the followingoperations if, and only if, the Boolean result is true; calculation bythe processor of a calculated second-duty-cycle value equal to theintended value of time-averaged light flux divided by time-averagedlight flux value F2; and provision by the processor to the rectangularpulse generator of an input representative of an intendedsecond-duty-cycle value the same as the calculated second-duty-cyclevalue and an input representative of an intended second-frequency valuethe same as a predetermined reference second-frequency value.
 24. TheLED light flux setting method of claim 21, further comprising:calculation by the processor, either from one or more stored values oftime-averaged light flux measure or using the stored values of thefitting constants, the time-averaged light flux value F2 expected whenthe first duty cycle is set to the predetermined minimum value and thesecond duty cycle is set to 100%; determination by the processor of aBoolean result, the Boolean result being true if the intended value oftime-averaged light flux is greater than zero and less than apredetermined fraction Y of time-averaged light flux value F2, and theBoolean result being false otherwise; performance of the followingoperations if, and only if, the Boolean result is true; calculation bythe processor of a calculated second-duty-cycle value equal to theintended value of time-averaged light flux divided by time-averagedlight flux value F2; calculation by the processor of a calculatedsecond-frequency value equal to the calculated second-duty-cycle valuedivided by a predetermined minimum time-period value; and provision bythe processor to the rectangular pulse generator of an inputrepresentative of an intended second-duty-cycle value the same as thecalculated second-duty-cycle value and an input representative of anintended second-frequency value the same as the calculatedsecond-frequency value.
 25. The LED light flux setting method of claim21, further comprising: determination by the processor of a Booleanresult, the Boolean result being true if the intended value oftime-averaged light flux is less than or equal to zero, and the Booleanresult being false otherwise; performance of the following operation if,and only if, the Boolean result is true; provision by the processor tothe rectangular pulse generator of an input representative of anintended first-duty-cycle value of zero or an input representative of anintended second-duty-cycle value of zero.